But by the rules stated, if their blood is identical, it should just keep the portal open twice as long, because as long as the blood flows into the portal then the portal stays open. Jumping into it should make it open real big, because tada, a whole person of blood!

I might be remembering this wrong, but what I took away from it is that the blood is more like on/off switch - it opens the portal, which then stays open until the blood stops flowing (i.e. until the person that it came from dies.) If the portal can't tell the difference between Buffy and Dawn, then it makes sense that Buffy dying would satisfy that requirement. And since the portal could (apparently) only be opened in that specific time and place, even if Dawn's still bleeding after that it wouldn't do anything. At least, that's the explanation that made the most sense to me.

(And yes, I am going through your archives and looking at your Buffy-related entries. I've been rewatching it over the past few months, and since none of my friends are into it quite as much as I am I've got some stuff to get out of my system.)